1. Field of the Invention
The present invention relates to an automatic focusing apparatus for automatically focusing an optical apparatus such as a camera.
2. Description of the Related Art
An automatic focusing apparatus of this type includes a photographic lens and a pair of lenses for re-focusing light beams which are transmitted through different portions of the photographic lens. This apparatus uses a so-called phase correlation scheme. In this scheme, focusing is performed on he basis of fact that the distance between images obtained by the pair of lenses varies depending on a defocus state. In addition, an automatic focusing apparatus using a so-called hill-climbing scheme is available. In this scheme, focusing is performed by detecting the contrast of an image formed by a photographic optical system, and moving the photographic optical system to a position where the contrast is maximum.
FIG. 24 is a view for explaining the principle of the phase correlation scheme. An optical system shown in FIG. 24 comprises: a photographic lens 1; a condenser lens 2, arranged on the focus side of the photographic lens 1, for receiving light emerging from the lens 1 as incident light; diaphragm masks 3 and 4 for respectively transmitting only the light beams emerging from the photographic lens which pass through peripheral portions A and B to be transmitted therethrough; a pair of separator lenses 5 and 6 for respectively focusing the light beams transmitted through the diaphragm masks 3 and 4; and image sensors 7 and 8 on which images are formed by the light beams focused by the separator lenses 5 and 6. Note that in FIG. 24, reference symbol O denotes an object to be photographed which is located on the optical axis of the photographic lens 1; and F, a predicted focal plane.
In the optical system having the above-described arrangement, if the object O is set at an in-focus position, the images of the object O are formed on the image sensors 7 and 8 by the separator lenses 5 and 6. In this case, a distance d between the images varies in accordance with a defocus state caused by light beams transmitted through the portions A and B of the photographic lens 1.
A defocus direction and a defocus amount can be obtained by calculating the positions of the two images on the image sensors 7 and 8 by a correlation operation, and obtaining the distance d.
FIG. 25 is a graph for explaining the principle of the hill-climbing scheme. The graph shows the contrast characteristics of an image near the predicted focal plane. As shown in FIG. 25, when the object O is set at the in-focus position, the image of the object O has the maximum contrast. If a defocus state (so-called front-focus or rear-focus state) occurs, the contrast level is lowered.
The optical lens, therefore, can be moved to the in-focus position by always moving the optical lens 1 in a direction in which the contrast level is increased.
According to another method, focus adjustment is performed by using the MTF (Modulation Transfer Function) characteristics of a photographic optical system. Published Unexamined Japanese Patent application No. 62-284314 discloses an automatic focusing apparatus using such a method. This apparatus comprises a storage means for prestoring MTF characteristics corresponding to defocus states at predetermined spatial frequencies, a means for obtaining spatial frequency components of images at least at three positions on the optical axis of a optical lens 1, and a means for determining the actual position of the optical lens by comparing the spatial frequency components obtained by the above-described means with the prestored MTF characteristics, and detecting a defocus amount.
According to still another method, an image is formed by a photographic optical system at two positions near a predicted focal plane, and focus adjustment is performed on the basis of focal point data obtained from the respective images. Published Unexamined Japanese Patent application No. 63-127217 discloses an automatic focusing apparatus using this method. This apparatus is designed to perform focus adjustment by calculating a defocus amount on the basis of the ratio between high-frequency components detected by a photographic optical system in two different focusing states.
In the above-described automatic focusing apparatuses, however, the following problems have been posed.
In the automatic focusing apparatus using the phase correlation scheme, if an object to be formed has a certain pattern, high correlation levels appear at a plurality of positions. Hence, an accurate position of the object cannot be obtained, and focusing adjustment cannot be performed. In addition, the apparatus requires a large number of components, such as the condenser lens 2, the diaphragm masks 3 and 4, the separator lenses 5 and 6, and the image sensors 6 and 7, as members specially used for focus adjustment. Therefore, it is difficult to reduce the apparatus in size. Besides, a high process/assembly precision is required. Furthermore, since only light beams which are transmitted through the portions A and B of the optical lens 1 are incident on the image sensors 7 and 8, the amount of incident light is relatively small. Therefore, the time required for focusing on a low-brightness object to be formed is undesirably prolonged. Moreover, the apparatus is subjected to the limitations of an optical system (an F-number, a reflecting telescope, and the like).
In the automatic focusing apparatus using the hill-climbing scheme, the optical lens is caused to pass through the focusing position once so as to detect a defocus direction and a defocus amount, and focusing adjustment is performed by moving the optical lens again. Therefore, the optical lens must be moved a plurality of times, and it takes a long period of time to focus the lens.
In the automatic focusing apparatus for performing focus adjustment using MTF characteristics as disclosed in Published Unexamined Japanese Patent application No. 62-284314, frequency components of an image are obtained at least three positions of the optical lens. Consequently, imaging and movement of the optical system must be performed at least three times, and focusing requires a long period of time. In addition, since only spatial frequency components of a single image are used, if the image includes only a small amount of spatial frequency components, the defocus detection precision is greatly reduced.
In the automatic focusing apparatus disclosed in Published Unexamined Japanese Patent application No. 63-127217, a defocus amount is calculated on the assumption that the MTF characteristics of the photographic optical system exhibit a Gaussian distribution. Therefore, a large error may be caused. In addition, since a defocus amount is calculated by using only spatial frequency components of a single image, if the image includes only a small amount of spatial frequency components, a larger error may be caused.